1. Field of the Invention
The present invention relates to a rotation angle detection apparatus, and more specifically relates to a rotation angle detection apparatus that has the function of detecting the entry of a foreign substance based on the frequency characteristic of signals.
2. Description of Related Art
Optical or magnetic detection devices having a plurality of signal tracks are used for measuring the angle of an output shaft of a motor or a table. As methods for detecting a malfunction in the detection device, a method in which a malfunction (alarm) in the detection device is detected when the amplitude of each signal is less than a specific threshold value (for example, Japanese Unexamined Patent Publication (Kokai) No. 2010-266260) and a method in which a malfunction (alarm) in the detection device is detected when signals of different phases have the same voltage value and an electrical angle at that time is different from a “normal electrical angle” stored in advance (for example, Japanese Unexamined Patent Publication (Kokai) No. 2005-147733) are known.
However, these methods detect a malfunction only after a motor or a machine tool equipped with the detection device breaks down. Preparing a maintenance component, after the malfunction has been detected, requires time for recovery of the motor or the machine tool, and therefore some maintenance components are often prepared in advance. However, the management of the maintenance components requires many man-hours and much storage space.
To solve the above problem, a method in which an abnormality in the amplitude of each signal is detected using a plurality of threshold values to anticipate the possibility of a malfunction before the detection device completely breaks down is known (for example, Japanese Unexamined Patent Publication (Kokai) No. 6-66594). However, there are various causes of malfunctions in the detection device, e.g. a break in a cable, electrical noise, and the entry of a foreign substance such as a cutting fluid, a lubricant, and chips into the detection device, thus making it difficult to specify a cause of the malfunction and carry out appropriate maintenance.